1. Field of the Invention
This invention relates to a newsprint paper for offset printing that offers excellent printing efficiency and print quality in offset printing.
2. Description of Related Art
Newsprint paper have become lighter by approx. 8 g/m2 over the past ten years or so, and some large users are now using extra-light newsprint papers weighting 40.5 g/m2. Use of color printing has also accelerated in the past several years, as the development of tower-press printing machines enabled double-face printing in color. As much as a half of all newsprint paper pages may be printed in color in a near future.
These trends reflect the strong demands for quality newsprint papers that are growing every year. In particular, there is a high demand for papers that can suppress show-through (lack of opacity of a printed paper; i.e., a phenomenon where the characters and illustrations printed on the other side are shown). Several methods are available to suppress show-through. Among them, the most effective method is to use pulps and/or fillers offering high specific scattering coefficients (a high specific scattering coefficient indicates less transmission of light). Among various pulps, mechanical pulp has a high specific scattering coefficient. However, the content of mechanical pulp has been decreasing of late due to an increase in the content of deinked pulp (DIP). As a result, suppressing show-through by means of changing the ratios of component pulps is becoming difficult. Because of this, increasing the content of fillers than that of fibers has emerged as an effective way to improve the opacity of paper. Accordingly, attempts have been made to increase the filler content in paper.
The DIP content in newsprint papers is increasing every year, as the users become more environmentally conscious and the paper manufacturers drive further cost reduction, among other reasons. Currently, it is not rare to find a newspaper containing over 70% of DIP. However, an increase in the DIP content results in various quality problems, such as reduced paper thickness and lower strength. In offset printing, a high DIP content can cause blurred ruled lines and roughness on solid areas due to paper powder deposit. Among these problems, paper powder deposit not only reduces print quality, but it also affects the printing efficiency because a large amount of paper powder deposited on the blanket cylinders of the press prolongs the cleaning time for the cylinders. Newspaper companies have successfully increased and continue to increase the printing speed and volume in recent years through adoption of online editing, advanced direct plate-making technology, etc. Therefore, conduciveness to printing efficiency is one of the key quality features that newspaper companies look for in material papers. A large amount of paper powder deposit necessitates frequent stopping of the press to clean the blanket cylinders. If the cleaning time becomes longer by several tens of minutes, the newspaper delivery will be affected and the readers will complain. This is why newspaper companies are particularly concerned about the problem of paper powder deposit.
As explained above, the most effective way to suppress show-through is to increase the filler content in the material newspaper. However, increasing the filler content generally lowers the surface strength and tensile strength of paper and also reduces the thickness of paper. In particular, lower surface strength allows more paper powder to deposit on the blanket cylinders of the web rotary offset press in offset printing, and the deposited paper powder causes blurred characters and ruled lines or roughness on solid areas (consequently poor ink impression). Normally, increasing the content of white carbon, talc, kaolin and other fillers in the material newsprint paper is known to increase the generation of paper powder. Since most of the ash content of DIP comes from fillers, the amount of paper powder entering the paper may also increase, in which case the higher powder content will cause other problems.
Generation of paper powder has been prevented by various means, such as blending pulps offering high surface strength, adding paper strengthening agents in the material mixture or coating oxidized starch on the produced paper. However, none of these methods can effectively suppress generation of paper powder.
For example, a technology has been presented in which 0.7 to 2.0 g/m2 of modified starch is applied on each side of the paper to reduce paper powder generation (refer to Publication of Unexamined Patent Application No. 2002-294587). If the amount of coated starch is increased, however, the damping water used in offset printing will add stickiness to the paper surface, which is undesirable. In addition, the effects of paper powder deposit on blurred ruled lines and poor ink impression on solid areas have not been evaluated accurately because of the absence of physical properties that can be used to control the generation of paper powder.